In order for a spacecraft or satellite to move on its own accord in free space, the spacecraft or satellite generally must carry some mass that it can accelerate in one direction so as to propel itself in an opposite direction. This mass, often termed the “reaction mass” or “propellant,” can be accelerated using a variety of means, including high temperature, fluid dynamics, electrostatic or electromagnetic forces, and the like.
Ion thrusters comprise one category of low-power engines where an electrostatic or electromagnetic force is used to accelerate an ionized reaction mass. Examples of ion thrusters include Hall or Hall effect thrusters, electrostatic ion thrusters, field emission electric propulsion (FEEP) thrusters, and colloid thrusters. This type of electric propulsion engine generally functions by ionizing the atoms of the reaction mass and then providing a voltage gradient to accelerate the ions out from the spacecraft or satellite. If too many accelerated ions in the exhaust plume are left as ions (i.e., charged), then a net build-up of charge will occur. This excess charge will alter the local electric field, causing a slowdown of ions that are accelerated in that region. Thus, after being exhausted from the engine of the spacecraft or satellite, the ionized reaction mass is neutralized.
Ion thrusters use an electron source to neutralize the ionized reaction mass. Most ion thrusters employ thermionic hollow cathodes, which require a gas flow in order to emit electrons. This propellant does not contribute to the thrust of the engine, yet still must be carried by the spacecraft or satellite. In addition, hollow cathodes can require a heater element, which is an additional load on the spacecraft or satellite power system. Since most low-power electric propulsion engines have limited power capacities, any expenditure in power (e.g., additional propellant needed for the hollow cathode, heat for the hollow cathode, or the like) that does not directly generate thrust is a source of inefficiency.
Accordingly, there remains a need for improved electron sources for neutralizing exhaust plumes in low-power electric propulsion devices. It would be particularly advantageous if the improvements reduced or eliminated power and mass expenditures that do not directly generate thrust. Such improved electron sources would provide more efficient electric propulsion devices.
It is to the provision of such electron sources and the resulting electric propulsion devices that the various embodiments of the present inventions are directed.